In tennis, as the skill of a player matures, the speed of travel of the ball increases and more and more "close calls" decide matches. On a random probability basis, it is typical to have five percent of all services and volleys in a match decided by balls striking boundary service, base and center lines. A judge linesman, player or observer may well have to determine in less than a blink of the eye whether a tennis ball was "in" or "out". In fact, at a horizontal velocity of 100 miles per hour (approximately 1,716 inches per second) the tennis ball travels across the width of the court's marking tape in approximately one-thousandth of a second. This is more than one-hundred times faster than the time it takes for the human eye to blink. It is thus apparent to even a casual observer, let alone judges and umpires, that the human senses alone are inadequate to accurately discern whether a tennis ball is still in play or not.
Heretofore, a number of devices have been proposed to overcome the aforementioned problem. However, to date, none of these devices has gained significant acceptance by the tennis community. For example, devices utilizing photo electric or infrared eyes have been developed, however, they suffer from cost disadvantages and player "black-outs" and "shadowing" of the tennis ball's trajectory. Devices have also been constructed which have utilized pneumatic components, but these devices are not attractive because their bulk affects the conduct of play. Self marking substances such as chalk lead to controversies over when the mark on the court or the ball occurred.
U.S. Pat. No. 3,854,719 issued to Supran; U.S. Pat. No. 3,883,860, issued to von Kohorn; U.S. Pat. No. 4,092,634, von Kohorn; and U.S. Pat. No. 4,109,911, issued to Van Auken disclose the concept of utilizing conductive elements on a tennis ball and on a tennis court playing surface to actuate visual and audio signals indictative of where the ball strikes the surface. Such systems require undesirable modification of a conventional tennis ball.
U.S. Pat. No. 3,810,148 issued to Karsten et al discloses a tennis court monitoring system in which a transmitter sends a pulsed beam of light at periodic intervals along a boundary line. A receiver senses the pulsed beam of light, and a control circuit detects the interruption of the beam. The control circuit discriminates between short duration interruptions caused by a permissible event such as a tennis ball landing on the line, and long duration interruptions caused by an impermissible event such as a server's foot landing on the service line. This system involves complicated optics and requires precise aiming of the beam.
U.S. Pat. No. 3,415,517 issued to Krist discloses a tennis court monitoring system which utilizes impact conductors that extend under selected boundaries and areas of a tennis court. Transducers such as piezoelectric crystal microphones are connected to the impact conductors for converting an impact into a signal which operates as indicator-detector to aid an umpire in judging a match. Impact conductors and transducers are also associated with the tennis net for detecting and indicating an impact thereon. This system requires numerous conductors to be embedded in the court. Also, it would seem that analog signals from the microphones generated by a ball and foot would be difficult to discriminate.
U.S. Pat. No. 3,774,194 issued to Jokay et al discloses a tennis court monitoring system in which a modified tennis ball carrying antenna coils interacts with magnetic fields emanating from the tennis court surface so that receiver circuitry can detect the location at which the tennis ball strikes the surface. The utilization of RF receivers leads to noise, tuning, and sensitivity problems.
U.S. Pat. No. 4,062,008 issued to Carlsson et al discloses a tennis court monitoring system in which pressure sensitive switches of the capacitor type define the boundaries of the tennis court. The distance between the plates in the capacitor type switches as well as the dynamic spring rate of the elastic material between the plates is adjusted to detect ball bounces and not foot impacts. When a tennis ball hits one of the switches on a boundary line, the change of capacitance causes a change of voltage across the capacitor. This voltage change is sensed by an indicator unit including an amplifier which amplifies pulses from the switch. A monostable multi-vibrator connected to the output of the amplifier has a change-over time which determines the duration of light and audio signals indicating the impact of a ball on the boundary line. Adjusting the capacitor switches to enable reliable discrimination between ball and foot impacts would be difficult in this system.
U.S. Pat. No. 3,982,759 issued to Grant discloses a tennis court line monitoring apparatus in which a pair of parallel pressure sensitive switches of the capacitor type respectively extend over, and outside of, a boundary line of a tennis court. The pair of switches are connected to a circuit including a pair of linear integrated monolithic timing circuits which in turn cause a lamp and buzzer to be activated if the tennis ball strikes the out of bounds one of the switches. The circuitry times the duration of the closing of the switches so that if a player merely steps on the out of bounds one of the switches the lamp and buzzer are not activated. The bouncing of a tennis ball off of the in bounds one of the switches inhibits the timing circuitry so that the lamp and buzzer are not activated even though the ball subsequently closes the out of bounds one of the switches.
The Grant patent also discloses circuitry for placing the system in a foot foul detection mode. In this mode the closing of one of the pressure sensitive switches prior to a microphone receiving an acoustic pick-up of the sound of a tennis ball being struck results in an indication to the linesman that a foot fault has occurred.
The Grant patent appears to disclose the possible utilization of a contact type pressure sensitive switches. Grant recognizes that in his system which utilizes capacitor type switches if a player is standing on a line, the hitting of the line by a ball is not detected. However, Grant indicates that this can be overcome by installing separate switching circuits of suitable length along a single boundary line. Thus, a considerable duplication of switches would be required. Furthermore, while not mentioned by Grant, such multiple switches would necessitate the utilization of multiple timing circuits or multiplexer circuitry.
In the Grant system, whether contact type or capacitor type pressure sensitive switches are utilized oscillation after impact will occur since the layer of material between the conductors is resilent. Thus, even after the tennis ball or the player's foot has impacted the switch, the detector circuitry will receive repeated pulses or variations in capacitance. The circuitry described in Grant does not appear to be adapted for discriminating between signals representative of the impact of a ball or foot on the one hand, and spurious signals on the other hand.
Also of general interest in this field, although of less pertinence than the patents already discussed, are U.S. Pat. No. 2,709,592 issued to McAvoy; U.S. Pat. No. 3,341,204 issued to McDannold; U.S. Pat. No. 3,492,440 issued to Cerbone et al; and British Pat. No. 1,234,083 issued to National Research Development Corporation.